I need to know how much items i consumed in a determined period, ie, 15 days. I can do it in a monthly basis, basically using the month function to extract the month and work from there, but with an arbitrary time (which is the average lead time from my supplier) dont know how to get a function to split the date list in chunks of 15 (or whatever) days.

You could just split text to colums using the "." and the " "(space) as separators. From this, you can filter just the period that you need using the "Filter" option. Then you can use a funcition to count values.
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DiogoMar 28 '12 at 16:12

Do you want chunks of 15 days starting from the first date in the list? Or from some other date (e.g., the first of the month)?
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ExcellllMar 29 '12 at 16:05